1. Field of the Invention
The present invention relates to a mixed flow turbine for use in compact gas turbines, turbochargers, expanders and so on.
2. Description of the Invention
There are constant demands to increase the efficiency of this type of mixed flow turbine.
The efficiency of a turbine is expressed relative to a theoretical speed ratio (=U/CO) that is the ratio of the circumferential speed U of a blade inlet to a maximum flow rate, that is, a theoretical speed CO, at which working fluid (gas) is accelerated in accordance with the temperature and the pressure ratio of a turbine inlet.
One factor that reduces the efficiency is incidence loss. This is caused by the incidence, which is the difference between the flow angle β of gas flowing to the leading edge of a blade and the blade angle βk of the leading edge. That is, when the incidence increases, inflow gas is separated by the leading edge, which increases incidence loss.
Since the whole cross section of a blade of a mixed flow turbine, when cut along a given radius from a rotation axis, including the vicinity of the leading edge is formed in a curve (like a parabola), mixed flow turbines can be designed so that the flow angle β and the blade angle βk come close to each other. Although this allows the incidence of, for example, a hub surface, to come close to zero, incidence occurs between the hub and the shroud.
That is, since the cross section of the blade of the mixed flow turbine, when cut along a radius from a rotation axis, is formed in a curve (like a parabola), the distribution 109 of the flow angle β changes linearly between the hub and the shroud, whereas the distribution 110 of the blade angle βk changes like a parabola, as shown in FIG. 20, so that the incidence at the central area of the blade height increases to the maximum.
Incidence loss due to an increase in incidence caused by the difference between the distribution of the flow angle β and the distribution of the blade angle βk conversely causes an increase in the loss of the mixed flow turbine, decreasing the efficiency thereof.
One example that reduces the decrease in efficiency is proposed in Japanese Unexamined Patent Application, Publication No. 2004-92498.
As shown in FIG. 18 or 19, it is configured such that a leading edge 103 of a moving blade 101 is formed in a convex shape toward the upstream side in the working-fluid flowing direction. Thus, as shown in FIG. 20, a flow angle distribution 115 forms a downwardly convex curve.
As a result, the blade angle distribution 110 of the blade angle βk and the flow angle distribution 115 of the flow angle β come close to each other, particularly at the center between the hub and the shroud, and the incidence becomes Ina. That is, since the incidence decreases by ΔIn, the incidence loss is decreased by a corresponding amount.
One example of a mixed flow turbine is that disclosed in Japanese Unexamined Utility Model Application, Publication No. Sho-62-79938, in which the scroll is separated into two, and working fluid is supplied from one or both of them at the same time so that the supply of the working fluid can be varied.
As shown in FIG. 21, it is configured such that a scroll 120 is separated into a first inlet channel 122 for supplying gas to the hub side of a leading edge 125 of a moving blade 124 and a second inlet channel 123 for supplying gas to the shroud side by means of a partition wall 121. When the quantity of gas is small, for example, it is supplied only through the first inlet channel 122, and when the quantity increases, it is supplied through the first inlet channel 122 and the second inlet channel 123.
Although the one disclosed in Japanese Unexamined Patent Application, Publication No. 2004-92498 exhibits the flow angle distribution 115 shown in FIG. 20 when gas is supplied to the leading edge 103 of the moving blade 101 in an ideal state, actual products cannot be expected to supply gas in such an ideal state.
That is, in general, a flow channel from a scroll 105 to the moving blade 101 extends radially, as shown in FIG. 18, or this flow channel is inclined in such a manner as to extend on the slopes of the shroud surface and the hub surface, as shown in FIG. 19.
Furthermore, a nozzle 107 having a vane-shaped blade is sometimes provided directly upstream of the moving blade 101.
In the case where gas is supplied radially to the moving blade 101, as shown in FIG. 18, the inclination angle δ of the hub-side flow becomes small, which reduces the effect of the upstream convex shape of the leading edge 103 by half and increases the flow angle β of the hub side, as shown in a flow angle distribution 102 in FIG. 20.
This increases the incidence, which is the difference between the blade angle βk and the flow angle β, thus posing the problem of increased loss on the hub side.
As shown in FIG. 19, in the case where gas is supplied at an angle to the moving blade 101, the deflection angle of the flow from the scroll 105 to the shroud at a meridian plane increases because the inclination angle of the shroud is large. Thus, when the deflection angle at the meridian plane increases, the boundary layer on the shroud surface expands at the leading edge of the moving blade 101, so that the flow angle β at the shroud side decreases. That is, the incidence increases in the opposite direction, as shown by the flow angle distribution 104 in FIG. 20, thus posing the problem of increased loss on the shroud side.
On the other hand, the one disclosed in Japanese Unexamined Utility Model Application, Publication No. Sho-62-79938 is configured such that the partition wall 121 is present, as a plate having a limited thickness, upstream from the leading edge 125 of the moving blade 124, which develops a wake 128 downstream from the trailing edge of the partition wall 121, thus increasing loss.
Since the gas from the first inlet channel 122 and the second inlet channel 123 is supplied at an angle to the leading edge 125 of the moving blade 124, a boundary layer 127 develops at the shroud-side wall and the hub-side wall of the leading edge 125, thus increasing loss.